I will say that in terms of sheer, punch-you-in-gut pathos, Deep Sleep Nine's "The Visitor" just blew me away. Yes, it's a quasi time travel story-- Jake Sisko watches his father disappear in a chrono-synclastic infundibulum accident and devotes his life to restoring him, succeeding finally as an old man by giving his own life. Powerful, powerful stuff, and for my money one of the best Trek eps ever.